In the art of electrophotography, including xerography, an electrophotographic material comprising a photoreceptor overlying a conductive substrate is electrically charged generally by means of a corona discharge, the charged material is exposed to light from an image and subsequently electrically charged particles are deposited selectively thereon so as to produce a visible image. When the electrophotographic material is subjected to an electric charging, the amount of charge deposited on the material varies with the conditions associated with the corona discharge, such as relative humidity, contamination of the charging wire or electrode, spacing between the surface of the photoreceptor and the corona generator, the voltage applied to the corona generator, ambient pressure and temperature. Since the above enumerated factors can vary with time, a variable photoreceptor charge may result with the attendant variation in the copy produced. Further, the thickness, dielectric constant and dielectric leakage of the photoreceptor can vary from receptor to receptor and as a function of time, also producing a variable photoreceptor charge.
A prior art technique for compensating for the variation of charge applied to the surface of a photoreceptor is disclosed in U.S. Pat. No. 3,586,908. The apparatus disclosed therein regulates the photoreceptor potential by comparing the output of an electrometer to a preset fixed potential value and generating a signal representing the difference therebetween the integrating the difference signal. The integrated difference signal causes a high voltage supply to supply a corresponding input voltage to a corona generator whereby the latter charges the photoreceptor surface to the preset fixed potential value. A switch may be interposed at the input terminal of the integrator to restrict the input to those times when a valid sample is disclosed, providing discontinuous sampling that may be required when plates or sheets, as opposed to drums, are being utilized as the photoreceptor support.
In the opposite sense, the "discontinuous sampling" technique disclosed, however, is not adapted for use when the photoreceptor is supported on a drum or the electrophotographic material comprises a continuous photosensitive web.
The use of a continuous control system as disclosed in the aforementioned patent does not allow the measurement of photoreceptor potentials at any point in the cycle due to delay time problems associated therewith. Further, the output of the corona generator is not periodically corrected for the duration of the cycle. The potential level per cycle therefore may be variable.
As exemplified in the aforementioned patent, the integrator utilized to integrate the difference, or error, signal is generally of the analog type, i.e., an operational amplifier having a feedback capacitor coupling the output to one of the amplifier inputs. Although analog integrators perform satisfactorily, long intervals between sampling increments may affect the charge retention capabilities of the integrating capacitor.